1. Field of the Invention
The present invention relates to a method and system for measuring sensitivity of electrophotographic photoconductors.
2. Discussion of the Background
The sensitivity of electrophotographic photoconductors is generally examined prior to shipment. The determination of sensitivity of photoconductors makes it possible to know whether the photoconductors can be used for specified copying machines or printers in consideration of their image forming conditions, such as illuminance and bias voltage, or what extent of illuminance and bias voltage is needed to record good images when the photoconductors are used therein. The sensitivity of photoconductors is generally represented by one of the following:
(1) an exposure amount (illuminance.times.exposure time) required when a surface potential of a photoconductor is decayed from a specified value to another specified value (unit: 1x.multidot.s); and PA1 (2) an amount of surface potential decay observed when a photoconductor is exposed to light whose exposure is predetermined (unit: volt). PA1 (1) charging a photoconductor with a charging device; PA1 (2) exposing the charged photoconductor to light which is preferably pulsed using a shutter; PA1 (3) obtaining changing information of the surface potential of the photoconductor using a surface potential meter while the photoconductor is exposed to light, and then obtaining a digitized changing curve of the surface potential of the photoconductor; PA1 (4) measuring illuminance (relative illuminance) on the surface of the photoconductor with a photo-sensor while the photoconductor is exposed to light, and then obtaining a digitized changing curve of the relative illuminance; PA1 (5) obtaining the exposure amount by integrating the changing curve of the relative illuminance; and PA1 (6) determining sensitivity, for example, as an exposure amount when the surface potential of the photoconductor is decayed from a specified value V1 to another specified value V2. PA1 (1) relative illuminance on the surface of the photoconductor is measured with a photo-sensor which is disposed between the shutter and the photoconductor while the photoconductor is exposed to light; and PA1 (2) a rising point of the changing curve of the relative illuminance is defined as a starting time of the exposure.
Typically, a photoconductor is generally exposed to light for a time on the order of a few seconds to examine its sensitivity.
Photoconductors mounted in analog copying machines are generally exposed to light for a time on the order of from tens of milliseconds to hundreds of milliseconds. Therefore, sensitivity of these photoconductors is needed to be examined with such a short pulse light to know whether the photoconductors can respond thereto. In this case, an exposure time is controlled so as to be, for example, 1/30, 1/60 or 1/125 second using a mechanical electromagnetic shutter.
Before sensitivity is examined, illuminance of light used for measuring sensitivity is generally measured using continuous light at the surface position of a photoconductor while opening a shutter. The thus obtained illuminance is multiplied by an exposure time to obtain an exposure amount.
In this case, since a mechanical shutter is used to control the exposure time, it takes a few milliseconds from a time when the shutter begins to open until the shutter is completely opened. In addition, it also takes almost the same time when the shutter is closed. Therefore, the exposure amount cannot be exactly measured unless such transition times are taken into consideration. In fact, background methods for examining sensitivity are imprecise particularly when the exposure time is relatively short.
Another method for measuring an exposure amount is used in which a sudden decay starting point of a light decay curve of surface potential of a photoconductor is considered to be a starting point of the exposure, the exposure time is defined as a period of time from the sudden decay starting point to a time when the surface potential becomes a predetermined surface potential, and the exposure amount is obtained as a product of the exposure time and the illuminance. However, among photoconductors for use in digital copying machines, there are photoconductors with a surface potential which does not suddenly decay soon after light is irradiated thereto and which suddenly decays after a certain amount of light is irradiated thereto. Therefore, this method for measuring an exposure amount is imprecise particularly when sensitivity of such photoconductors is measured.
In attempting to exactly determine a starting time of exposure, several methods have been proposed. One of them is to define the starting time as a time when the shutter opening operation is ordered. However, this method has a drawback in that it takes a considerable time from the order until the shutter actually begins to start. Another method is to define the starting time as a time when a signal that a shutter is in an opened state is output by a contact point of the shutter. However, this method has a drawback in that there are variations in the time when the signal is output because this method also uses a mechanical mechanism. In addition, these methods also have the above-mentioned drawback that an exposure amount during shutter opening and closing operations cannot be exactly determined.
Further, another method has been proposed in which the relationship between illuminance and a detected current of a photo-sensor is previously obtained while using an integral circuit to determine exact exposure amount. Even in this case, a starting time of exposure cannot be exactly determined, and therefore the exposure amount cannot be exactly determined.